Exchange 2003 Server Queue Filling up!

I've read every article and have done practically everything. I've made sure there isn't an open relay, I've applied Recipient Filtering ( Checked "Filter recipients who are not in the Directory" ), Sender Filtering, Connection Filtering, Sender ID Filtering, I've gone to Default SMTP Virtual Server - Access - Relay and ticked "Only the list below" and unchecked "Allow all computers which successfully authenticate to relay regardless of the list above" as well as left the "Computer list" blank under "Only the list below",  I've disabled port 25 on the firewall, changed the admin password, disabled accounts that weren't being used, enabled logging on NDRs and haven't seen the event id 1708 (to see if another account has been compromised), enabled Tar Pit, I've tested to make sure it isn't a DNS problem, and disabled NDR....

I'm still continuing to get hundreds of thousands of NDRs from unrecognized email accounts in the queue. I have no idea what to do to solve this problem :-( If someone could please point me in the right direction I'd appreciate it!